The present invention relates to an improved comb-line antenna structure that may be used to launch magnetosonic waves into a plasma for plasma heating or current drive in a plasma device, e.g., a tokamak. More particularly, the invention relates to an improved comb-line antenna structure suitable for use in a tokamak that, in one embodiment, eliminates the need for discrete loading capacitors at the end of each current strap used in the comb-line antenna, and provides Faraday shielding between the antenna and plasma in a way compatible with a traveling wave.
Tokamaks are devices that are used in connection with the study and generation of thermonuclear fusion energy. Fusion is the energy source of the sun and other stars. While science has not yet advanced sufficiently far to allow fusion to be used as a practical energy source, scientists and engineers, working at laboratories around the world, are making great strides relative to fusion research and to the engineering development of fusion for electrical power and other applications. Advantageously, fusion fuel is in abundant supply, and the generation of fusion energy provides a safe and clean energy source.
In generating fusion energy, the atoms of two or more fuels, typically deuterium (.sup.2 H) and tritium (.sup.3 H), heavy hydrogen isotopes, are exposed to extremely high temperatures. Such high temperatures separate the positively charged nuclei of the hydrogen isotopes from their normally tightly bound negatively charged electrons, forming a plasma. (A plasma is a hot ionized gas.) When this separation occurs, the neutrons and protons of the nuclei recombine to form a heavier element, such as .sup.4 He, and a neutron or other small nuclear particle. Energy from this reaction is released as kinetic energy of the fast moving reaction products, and it can be converted to heat. The heat thus created provides the high temperature needed to sustain the fusion reaction, and portions thereof can be extracted and used as a useful energy source, e.g., to generate electricity.
The conditions for the fusion reaction are very difficult to achieve. For example, in order to kindle a deuterium-tritium fusion fire, the temperature of the fuel must be heated to over 50,000,000.degree. C. Moreover, to sustain the fusion fire, i.e., to keep the fusion reaction going, it is necessary to confine the normally chaotic mass of fast moving, superheated nuclei (the plasma) long enough for the fuel to react and produce energy beyond that necessary to sustain the temperature. To produce enough fusion reactions to make the process worthwhile, the heat losses from the fuel must be low enough so that the fuel can sustain a temperature of around 150,000,000.degree. C. Once such a self-sustaining reaction is achieved, it is possible to use the heat thus produced to generate electricity, or for other purposes.
Achieving such high temperatures requires supplying energy to the fuel and raising its temperature to a level where the internal fusion reactions can provide further heating. Various techniques are currently used to accomplish such heating, e.g., heating with an internal electric current, heating by various waves, and/or heating by the injection of energetic neutralized hydrogen atoms ("neutral beam injection"). The present invention relates to a particular type of antenna structure, referred to as a "comb-line antenna" structure, that may be used to launch a particular type of fast electromagnetic wave, referred to as a "magnetosonic wave", into the forming plasma for plasma heating and current drive.
Unlike the sun and stars, where the massive plasma ball is confined by gravity, fusion reactors require some type of container for holding the 150,000,000.degree. C. plasma fireball in a way that prevents it from touching the container walls. (Plasma, which has a density approximately 100,000 times lower than atmospheric pressure, is a mere puff of gas that would quickly cool if it touched the container walls.)
Fortunately, because plasma is an ionized gas, it can be confined with a magnetic field. That is, the otherwise random motion of the charged particles that are found within plasma may be converted to an orderly form of motion that follows the magnetic field lines of an applied magnetic field. Thus, various types of "magnetic bottles" have been developed in the art to create the appropriate magnetic field lines to confine the plasma to a desired volume.
One of the most highly developed magnetic bottles is a toroidal bottle known as the "tokamak". Tokamaks were first developed during the 1960s in the then-existing USSR, and have subsequently been adopted as the leading magnetic confinement device. A tokamak includes both external toroidal-field coils and poloidal-field coils that generate magnetic fields, as well as means for generating a toroidal electrical current that flows through the plasma itself. The magnetic fields created by such toroidal- and poloidal-field coil currents, as well as by the plasma electric current, all combine to confine the plasma to a general toroidal shape that encircles a major axis of the tokamak. The poloidal-field coils are also used to magnetically shape the general cross section of the plasma. Tokamaks are well documented in the literature. See, e.g., Artsimovich, L. A., Nuclear Fusion, Vol. 12, pp. 215 et seq. (1972); and Furth, H. P., Nuclear Fusion, Vol. 15, pp. 487 et seq. (1975).
The fast magnetosonic wave is a preferred wave for noninductive current drive in tokamaks containing high temperature plasmas. See, Fisch et al., Phys. Fluids, Vol. 24, p. 27 (1981). Note, the term "magnetosonic" wave is used herein and in the art to describe a "fast wave" or a compressional "Alfen wave." The launching structure used to launch such a magnetosonic wave into the plasma has typically consisted of several (typically four) poloidal straps that are individually fed through external matching networks and phase shifters to an appropriate power source (a high power rf generator). Disadvantageously, such a launching configuration does not present a matched load to the generator when the plasma position and edge density vary. Further, the mutual coupling between the current straps causes unbalanced loading of the straps. Such unbalanced loading greatly complicates the phasing and matching of the structure to the generator. Hence, what is needed is a launching structure for use in a tokamak (or similar plasma-confining device) that presents a matched load to the generator even though the plasma position and edge density of the plasma may vary, and wherein any mutual coupling between the current straps may be used to an advantage rather than a disadvantage.
A "comb-line" structure is a structure that includes an array of antenna loops, or equivalent current straps, where only one of such loops or straps is driven with an input signal, while the others are coupled through mutual inductance. Comb-line structures have long been used as band-pass filters having a narrow or moderate bandwidth. See, e.g., Matthaei, George L., "Comb-line Band-Pass Filters of Narrow or Moderate Bandwidth:, Microwave Journal, pp. 82-91 (August 1963).
A few years ago, the comb-line structure was identified as a possible launching structure for launching the ion cyclotron range of frequencies (ICRF) into the plasma of a fusion tokamak for plasma heating and current drive. See, Chiu et al., "Study of the Slow-Wave Structure as an ICRF Launcher", Nuclear Fusion, Vol. 24, No. 6, p. 717-723 (1984). The comb-line structure is a good candidate for current drive because it can easily be made to produce a traveling wave spectrum, and the principle of its operation requires that it be only weakly coupled to the plasma. Weak coupling is plausible for reactor applications because it allows the structure to be recessed from the plasma.
Despite the potential benefits to be derived from using a comb-line structure to launch magnetosonic waves into a plasma, the comb-line structure has not heretofore been used for such application. Such non-use can be attributed, in large part, to practical considerations associated with the dimensions of the various elements of the comb-line structure. For example, discrete capacitors have heretofore been used with each current strap in order to allow each current strap, which must function as a resonant circuit, to be of a manageable length. The physical size of such capacitors, while having a capacitance value of only tens of picofarads for most frequencies, must nonetheless be quite large so that the capacitors can hold off the extremely high voltages in the plasma environment. Such large physical size restricts the spacing of the current straps. Hence, what is needed is either a physically smaller capacitor that can hold off the extremely high voltages in the plasma environment, or a comb-line antenna structure that eliminates the need for discrete capacitors.
An additional problem associated with using a comb-line structure as a launching structure with a plasma device, such as a tokamak, is that of providing shielding of the plasma from the electrostatic fields emanating from the current straps. An electrostatic field is ever present around the current carrying straps of a comb-line structure. Such electrostatic field must be confined to the region near the current straps in order to avoid its penetration into the plasma, which penetration can cause an influx of impurities into the plasma. The appropriate magnetic fields associated with the current straps, on the other hand, need to encircle the adjoining straps so that the desired mutual coupling between the straps can occur, and so that the requisite magnetosonic wave can be launched into the plasma. Thus, there is a need when using a comb-line structure as a magnetosonic launcher for establishing an effective electrostatic shield between the current straps and the plasma that blocks electrostatic fields, yet passes traveling magnetic fields.